1. Field of the Invention
This invention relates to a liquid crystal composition for use in liquid-crystal displays of the field-effect type which are driven in a time-division fashion.
2. Description of the Related Art
Liquid-crystal displays are commonly used in various items of office equipment. It is demanded that a liquid-crystal display have a large screen, have many pixels, and display high-quality images. The most popular of these liquid-crystal displays is a simple-matrix type which not only has a simple structure but also can be driven by a relatively simple driver.
The liquid-crystal display of the simple-matrix type has a liquid-crystal cell. The liquid-crystal cell comprises a first substrate, a second substrate located parallel to the first substrate and spaced apart therefrom, stripe-shaped signal electrodes formed on the inner surface of the first substrate, stripe-shaped scanning electrodes formed on the inner surface of the second substrate and opposing and intersecting with the signal electrodes, and a liquid crystal twisted and sealed within the gap between the first and second substrate, forming a layer. Those portions of the liquid-crystal layer which are located at the intersections of the signal electrodes and the scanning electrodes function as pixels. The liquid-crystal display further has a pair of polarizing plates which sandwich the liquid-crystal cell.
To drive the liquid-crystal display, scanning signals are supplied to the selected ones of the scanning electrodes, and data signals are simultaneously supplied to the selected ones of the signal electrodes. As a result, those pixels which are located at the intersections of the selected scanning electrodes and the selected signal electrodes thereby are driven and allow the passage of light. This is how the liquid-crystal display is driven in a time-division fashion.
The more frequently the liquid-crystal display is driven in a time-division fashion, the smaller the operating margin of the display, i.e., the difference between the voltage applied to any pixel driven and the voltage applied to any pixel not driven. The smaller the operating margin, the lower the contrast of the image the liquid-crystal display displays, and, hence, the less the view angle the display has.
The conventional TN-type liquid-crystal display has but insufficient steepness in the changes of luminance which occur when voltages are applied to the pixels. The display is therefore driven at a time-division duty of, at most, 1/60. Consequently, it can not have so many pixels as to form a large screen or to display high-quality images.
Other types of liquid-crystal displays have been developed and put to practical use, which exhibit a greater steepness in the changes of luminance since the molecules of the liquid crystal are twisted by a larger angle than in the TN-type liquid-crystal display. They are generally known as a STN-type or a SBE-type, wherein the liquid crystal molecules have a twist angle ranging from 180.degree. to 270.degree.. The STN-type liquid-crystal display can be driven at a time-division duty of up to 1/200. However, this time-division duty is not sufficient for the 640.times.400 dot liquid-crystal display which has hitherto been used in great numbers. As a matter of fact, the 640.times.400 dot display consists of two sections, i.e., the upper and the lower, which are each driven at a time-division duty of 1/200.
Conventional liquid crystal compositions for use in liquid-crystal displays which are driven in a time-division fashion has four features, which enable the compositions to acquire sufficient steepness in the changes of luminance. The first is the great elastic constant ratio K.sub.33 /K.sub.11, i.e., the ratio of bend elastic constant K.sub.33 to splay elastic constant K.sub.11. The second is the small dielectric constant ratio .DELTA..epsilon./.epsilon..perp., i.e., the ratio of dielectric anisotropy .DELTA..epsilon. to perpendicular dielectric constant .epsilon..perp. (namely, the dielectric constant measured in a line perpendicular to the long axis of liquid crystal molecules). The third feature is prominent dielectric anisotropy .DELTA..epsilon.. The fourth feature is the low viscosity.
More specifically, the liquid crystal composition for use in the conventional STN-type liquid-crystal display is a mixture of at least three liquid crystal compounds. The first compound, the content of which is 50% or more, has a cyano group and serves to increase the elastic constant ratio K.sub.33 /K.sub.11 and also to decrease the voltage for driving the STN-type liquid crystal display. The second liquid crystal compound exhibits has ester bonds and a .epsilon..perp. value great enough to reduce the dielectric constant ratio .DELTA..epsilon./.epsilon..perp.. The third liquid crystal compound has low viscosity for adjusting the viscosity of the liquid crystal composition to a desired value.
The liquid crystal composition used in the conventional STN-type liquid-crystal display has drawbacks. Its first component, i.e., the liquid crystal compound having a cyano group, has high viscosity, inevitably increasing the viscosity of the liquid crystal composition. Its second component, i.e., the liquid crystal compound having ester bonds and a great .epsilon..perp. value for reducing the ratio .DELTA..epsilon./.epsilon..perp., also has high viscosity, degrading the response speed of the liquid crystal cell. Moreover, the liquid crystal composition fails to form images of uniform contrast after a long, continuous use of the liquid-crystal display.